A polarizing plate used for glasses (or spectacles), sunglasses, goggles and others comprises a polarizing membrane (or film) and a protective film for the polarizing membrane. For the protective film in recent years, the requirements for the properties (or characteristics) such as optical properties, mechanical properties and chemical resistance have been strict. Moreover, with respect to the recent glasses and the like, the requirement not only for practical functions but also for fashionability with an aesthetic value (e.g., frameless glasses) has been strong.
Thereat, protective films for a polarizing membrane with various excellent properties have been developed. For example, Japanese Patent Application Laid-Open No. 305341/2001 (JP-2001-305341A, Patent Document 1) discloses a polarizing plate comprising a polarizing membrane and polycarbonate resin (PC) sheet (s) laminated on one side or both sides thereof as a protective film for the polarizing membrane. This document mentions that the protective film has a retardation value (Δn·d) of not more than 300 nm, which is defined as a product of a birefringence index (Δn) and a thickness (d), and the protective film ensures a sufficient polarization degree without generating a degree in the case of forming the protective film into a curved surface form such as a spherical surface and without generating a crack on the PC sheet.
However, since the polycarbonate resin has a high dispersibility (a low Abbe number), the protective film generates light chromatic dispersion resulting in light blurring, and tends to generate chromatic aberration when the protective film is observed from an oblique angle. Moreover, in the case of punching a hole or forming a tapped hole in the polarizing plate for preparing frameless glasses or sunglasses, the protective film comprising the polycarbonate resin sometimes generates a fracture or a crack around the hole and has photoelasticity. Therefore, it is impossible to certainly impart a high moldability to the protective film. Further, the document describes that the protective film is excellent in solvent resistance, and that, in the case of pasting the polarizing membrane and the polycarbonate resin sheet with an adhesive, the protective film can prevent or inhibit generation of a crack on the sheet. However, a frame for glasses or the like is formed from aplastic resin containing a plasticizer (e.g., a cellulose acetate) in many cases. There is a possibility that such a plasticizer bleeds out and moves to the polarizing plate to generate a crack on the protective film comprising the polycarbonate resin.
Japanese Patent Publication No. 2838514 (JP-2838514B, Patent Document 2) discloses a polarizing plate comprising a polarizing membrane and polycarbonate resin sheet (s) laminated on one side or both sides thereof as a protective film for the polarizing membrane, as is the case with Patent Document 1. The protective film has a retardation value (Δn·d) of 3000 to 6000 nm. This document mentions that, by increasing the retardation value in the protective film, it is hard to generate chromatic aberration developed in the case of bending the protective film into a lens form. However, in the protective film described in this document, it is difficult to sufficiently inhibit or prevent development of irregular color in a bending. In addition, the protective film is deteriorated in solvent resistance so that the film is sometimes corroded by a solvent for an adhesive.
As described above, a conventional protective film for a polarizing membrane has had problems such as a light leakage phenomenon or irregular color development generated in a bending, crack development due to insufficient solvent resistance, and turbidity.
On the other hand, Japanese Patent Application Laid-Open No. 189199/2002 (JP-2002-189199A, Patent Document 3) discloses a polarizing molded article comprising a polarizing plate-containing laminated structure in which a polarizer sheet layer is held between two protective sheet layers, wherein one layer of the protective sheet layer and a polyurethane sheet layer or a polyamide sheet layer are connected with an adhesive or a pressure sensitive adhesive, and the polyurethane sheet layer or the polyamide resin sheet layer and a thermally molding resin layer are thermally adhered. This document also mentions that the polyamide sheet layer is a transparent polyamide sheet and that the resin constituting the thermally molding resin layer is a polyamide (e.g., a transparent polyamide), a thermoplastic polyurethane, or a polycarbonate. Incidentally, in this document, there is no description about a light leakage phenomenon or irregular color.
Moreover, Japanese Patent Application Laid-Open No. 257403/1985 (JP-60-257403A, Patent Document 4) discloses a polarizing plate comprising a polarizing film, and an oriented film which is drawn to one direction parallel to a film surface thereof so as to satisfy a specific relationship in refractive index and which is bonded to at least one side of the polarizing film through an adhesive layer, and discloses that the oriented film is selected from the group consisting of a polysulfone film, a polycarbonate film, a polyamide film, and a polyether sulfone film. The examples of the document mentions that a uniaxially oriented polycarbonate film having a thickness of 100 μm with a specific relationship in refractive index was produced by heating an unoriented polycarbonate film having a thickness of 300 μm to 130° C., and three-fold drawing the film to one direction (Y direction). Moreover, this document describes that a polarizing plate which generates no colored interference fringe [irregular color caused by optical anisotropy (birefringence index) due to a drawing process] is obtained by using such a film satisfying a specific refractive index alone or using two films which are arranged so that these optical axes are set in perpendicular to each other. Incidentally, also in this document, there is no description about a light leakage phenomenon.    [Patent Document 1] JP-2001-305341A (Claims, and Paragraph numbers [0012] and [0014])    [Patent Document 2] JP-2838514B (Claims, and Paragraph numbers [0005] and [0006])    [Patent Document 3] JP-2002-189199A (Claims)    [Patent Document 4] JP-60-257403A (Claims, Examples, the upper left column of page 2, and the lower left column of page 5)